A pulse oximeter works by shining two beams of light through your skin and measuring how much light your blood absorbs. Because oxygen-rich blood and oxygen-poor blood absorb light differently, the device can calculate the percentage of your hemoglobin carrying oxygen, a reading called SpO2. A normal reading is 95% or higher.
Two Wavelengths of Light Do the Heavy Lifting
Inside every pulse oximeter are two tiny LEDs. One emits red light at 660 nanometers, and the other emits infrared light at 940 nanometers. These two specific wavelengths were chosen because oxygenated hemoglobin and deoxygenated hemoglobin absorb them in opposite ways. Hemoglobin carrying oxygen absorbs more infrared light and lets more red light pass through. Hemoglobin without oxygen does the reverse, absorbing more red light and letting more infrared through.
On the other side of your finger (or earlobe, or toe), a photodetector sits directly opposite the LEDs. It measures how much of each wavelength made it through your tissue. The device then compares the ratio of red light absorbed to infrared light absorbed, and from that ratio, it calculates your oxygen saturation percentage.
How It Isolates Your Arterial Blood
Your finger contains arteries, veins, bone, skin, and other tissue, all of which absorb some light. So how does the device know it’s reading arterial oxygen specifically? The answer is your pulse. Every time your heart beats, a small surge of arterial blood expands the tiny vessels in your fingertip. That expansion causes a brief, rhythmic increase in light absorption. The device detects this pulsing signal and separates it from the constant, unchanging absorption of everything else: veins, bone, skin, and surrounding tissue.
This is why it’s called a “pulse” oximeter. It only analyzes the fluctuating component of the signal, which corresponds to fresh arterial blood. It’s also why the device displays your heart rate alongside your oxygen level. Both readings come from the same pulsatile signal.
The Physics Behind the Calculation
The underlying principle is a rule from physics called the Beer-Lambert law. In simple terms, it says that when light passes through a substance, the amount of light absorbed depends on two things: how concentrated the substance is and how far the light has to travel through it. A thicker layer of a substance absorbs more light. A more concentrated solution absorbs more light. The relationship is exponential, meaning small changes in concentration can produce measurable shifts in how much light gets through.
In a pulse oximeter, the “substance” is hemoglobin in your blood. The device uses the known absorption properties of oxygenated and deoxygenated hemoglobin at 660 and 940 nanometers, plugs in the ratio detected by the photodetector, and outputs a percentage. A ratio that shows high infrared absorption relative to red absorption means most of your hemoglobin is carrying oxygen.
Where the Sensor Goes and Why
The fingertip is the standard placement because it’s thin enough for light to pass through, has a dense network of small blood vessels, and produces a strong pulsatile signal. The probe clips onto your finger with the LEDs on one side and the detector on the other, creating a direct light path through your tissue.
Other placement sites include the earlobe, toe, and forehead. Earlobe sensors work on the same transmissive principle as finger clips. Forehead sensors use a slightly different approach: both the light source and detector sit on the same side of the skin, and the device measures light reflected back rather than light transmitted through. These alternative sites are useful when a finger reading isn’t possible, for example during surgery when the hands aren’t accessible, or when poor circulation in the extremities weakens the finger signal.
What Can Throw Off a Reading
Pulse oximeters are reliable in most conditions, but several factors can interfere with accuracy.
- Dark nail polish. Black and brown nail polish can block enough light to prevent a reading entirely. In one study, only 12% of fingers with black nail polish and 64% with brown polish registered a saturation value at all. When readings did appear, they were significantly lower than the true value. Red, clear, and acrylic nails had minimal effect.
- Poor circulation. Cold hands, low blood pressure, or conditions that reduce blood flow to the fingers weaken the pulsatile signal the device depends on. Without a strong pulse, the oximeter may display erratic numbers or fail to get a reading.
- Movement. Shivering, trembling, or moving your hand while the sensor is attached creates signal noise that the device can misinterpret as changes in light absorption.
- Skin pigmentation. This is one of the most clinically significant accuracy issues. A 2025 study in the BMJ found that pulse oximeters overestimated oxygen levels by 0.6 to 1.5 percentage points in patients with darker skin tones compared to lighter skin tones. That may sound small, but it has real consequences. The rate of false negatives (readings that appeared normal when oxygen was actually dangerously low) was 2.3 to 7.1 times higher in patients with darker skin. The FDA issued draft guidance in January 2025 acknowledging this problem and recommending that manufacturers specifically test devices across a range of skin pigmentations.
What the Numbers Mean
A reading of 95% or higher is normal for most people. This means at least 95 out of every 100 hemoglobin molecules passing through your fingertip are carrying oxygen. Some people with chronic lung conditions or sleep apnea may have a normal baseline closer to 90%, but for most adults and children, anything consistently below 95% warrants attention.
The device is measuring peripheral oxygen saturation (SpO2), which is an estimate of arterial oxygen saturation (SaO2). In healthy individuals, the two numbers are very close. But in critical illness, severe anemia, or carbon monoxide poisoning, SpO2 can diverge from the true arterial value. Carbon monoxide binds to hemoglobin in a way that looks identical to oxygen at the wavelengths the device uses, so a pulse oximeter will show a falsely normal reading in someone with carbon monoxide poisoning.
Home Devices vs. Medical-Grade Units
Over-the-counter finger pulse oximeters use the same basic technology as hospital models. The difference is in calibration precision, build quality, and regulatory testing. Medical-grade devices sold for clinical use go through FDA review that includes testing across a range of oxygen levels and, increasingly, across different skin tones. Consumer devices sold as “wellness” products may not undergo the same scrutiny.
For general home monitoring, a consumer pulse oximeter gives a useful ballpark. To get the most accurate reading, make sure your hands are warm, remove dark nail polish, sit still, and wait for the number to stabilize for at least 10 to 15 seconds before reading it. If readings seem inconsistent, try a different finger. The index and middle fingers tend to give the strongest signal.